Many measurement instruments receive data in analog form and convert it to digital form for analysis and display using an analog-to-digital converter (ADC). The incoming signals typically need to be amplified and filtered prior to reaching the ADC. Current and past amplifiers have some shortcomings that can be improved.
FIG. 1 shows a prior art gain stage, often referred to as a gm-RL amplifier having two transistors 10 and 12, referred to here as gain transistors. The gain is proportional to the tail current I, and the gain being equal to the product of the gm and the RL. Most transistors suffer from a shortcoming in that there is an ohmic resistance associated with the base-emitter loop. There is a resistance from the base to the emitter, including the intrinsic base and a contacting resistance from the intrinsic emitter to the contact. This results in the gain not being exactly what one would expect based upon the current, because these resistances lower it. In addition, the response is non-linear when the input signal swings.
FIG. 2 shows an example of a translinear amplifier having a series of transistors as loads. Transistor 12 has transistors 16 and 14 as loads, and transistor 10 has transistors 18 and 20 as loads. In this example, all of the ohmic components are the same. This results in the gain being 2, referred to as an ×2 amplifier, being a numerical ‘times 2’ amplification. The gain also does not depend upon the current. For at least a portion of the response of the amplifier in FIG. 2, the response is linear because whatever happens in the gain stage happens in the load as well. However, the increase in re means that the currents get lower. As the currents get lower, the bandwidth suffers. The amplifier provides an output to drive the load that will have a capacitive aspect to it. A high re which is seen by the transistors when the current goes down results in a non-linear response in the lower current ranges.
It is possible to develop an amplifier that has performance characteristics in between the two types of amplifiers.